The present invention is a catalyst structure and method of making, and a method of Fischer-Tropsch synthesis.
Fischer-Tropsch synthesis is carbon monoxide hydrogenation that is usually performed on a product stream from another reaction including but not limited to steam reforming (product stream H2/COxcx9c3), partial oxidation (product stream H2/COxcx9c2), autothermal reforming (product stream H2/COxcx9c2.5), CO2 reforming (H2/COxcx9c1) coal gassification (product stream H2/COxcx9c1) and combinations thereof.
Fundamentally, Fischer-Tropsch synthesis has fast surface reaction kinetics. However, the overall reaction rate is severely limited by heat and mass transfer with conventional catalysts or catalyst structures. The limited heat transfer together with the fast surface reaction kinetics may result in hot spots in a catalyst bed. Hot spots favor methanation. In commercial processes, fixed bed reactors with small internal diameters or slurry type and fluidized type reactors with small catalyst particles ( greater than 50 xcexcm) are used to mitigate the heat and mass transfer limitations. In addition, Fischer-Tropsch reactors are operated at lower conversions per pass to minimize temperature excursion in the catalyst bed. Because of the necessary operational parameters to avoid methanation, conventional reactors are not improved even with more active Fischer-Tropsch synthesis catalysts. Detailed operation is summarized in Table 1 and FIG. 1.
Literature data (Table 1 and FIG. 1) were obtained at lower H2/CO ratio (2:1) and longer residence time (3 sec or longer). Low H2/CO (especially 2-2.5), long residence time, low temperature, and higher pressure favor Fischer-Tropsch synthesis. Selectivity to CH4 can be significantly increased by increasing H2/CO ratio from 2 to 3. Increasing residence time also has a dramatic favorable effect on the catalyst performance. Although reference 3 in Table 1 shows satisfactory results, the experiment was conducted under the conditions where Fischer-Tropsch synthesis is favored (at least 3 sec residence time, and H2/CO=2). In addition, the experiment of reference 3 was done using a powdered catalyst on an experimental scale that would be impractical commercially because of the pressure drop penalty imposed by powdered catalyst. Operating at higher temperature will enhance the conversion, however at the much higher expense of selectivity to CH4. It is also noteworthy that residence time in commercial Fischer-Tropsch units is at least 10 sec.
Hence, there is a need for a catalyst structure and method of Fischer-Tropsch synthesis that can achieve the same or higher conversion at shorter residence time, and/or at higher H2/CO.
The present invention includes a catalyst structure and method of making the catalyst structure for Fischer-Tropsch synthesis that both rely upon the catalyst structure having a first porous structure with a first pore surface area and a first pore size of at least about 0.1 xcexcm, preferably from about 10 xcexcm to about 300 xcexcm. A porous interfacial layer with a second pore surface area and a second pore size less than the first pore size is placed upon the first pore surface area. Finally, a Fischer-Tropsch catalyst selected from the group consisting of cobalt, ruthenium, iron, rhenium, osmium and combinations thereof is placed upon the second pore surface area.
Further improvement is achieved by using a microchannel reactor wherein the reaction chamber walls define a microchannel with the catalyst structure placed therein through which pass reactants. The walls may separate the reaction chamber from at least one cooling chamber.
The present invention also includes a method of Fischer-Tropsch synthesis having the steps of:
(a) providing a catalyst structure having a first porous structure with a first pore surface area and a first pore size of at least about 0.1 xcexcm;
a porous interfacial layer with a second pore surface area and a second pore size less than the first pore size, the porous interfacial layer placed upon the first pore surface area;
a Fischer-Tropsch catalyst selected from the group consisting of cobalt, ruthenium, iron rhenium, osmium and combinations thereof placed upon the second pore surface area; and
(b) passing a feed stream having a mixture of hydrogen gas and carbon monoxide gas through the catalyst structure and heating the catalyst structure to at least 200xc2x0 C. at an operating pressure, the feed stream having a residence time within the catalyst structure less than 5 seconds, thereby obtaining a product stream of at least 25% conversion of carbon monoxide, and at most 25% selectivity toward methane.
It is an object of the present invention to provide a catalyst structure for Fischer-Tropsch synthesis.
It is another object of the present invention to provide a method of Fischer-Tropsch synthesis having shorter residence time.
Advantages of the invention include (i) at residence time shorter than the prior art, higher conversions are achieved with no increase to methane selectivity; and (ii) as residence times increase, conversion increases and methane selectivity decreases (slightly). Surprisingly, the present invention represents an increase in conversion efficiency of at least a factor of 3 on the basis that equivalent conversion with conventional catalyst would require correspondingly greater residence time.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.